As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks have to project smaller and smaller structures. In order to fulfill this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In order to use significantly smaller wavelength lithography system for the extreme ultraviolet (EUV) wavelength range (approximately 13.5 nm) are presently in development.
Photolithographic masks have to fulfill highest demands with respect to transmission, planarity, pureness and temperature stability. In particular, the surface of reflective masks for EUV radiation coated with the reflective structure has to be plane within the range of about 1 nm in order to avoid aberrations of the desired structure in the photoresist of the wafer. These challenges also apply for other EUV reflective optical elements e.g. mirrors used in the beam path of EUV lithography (EUVL) systems.
Well known methods exist for the final precision polishing of the optical surfaces (J. S. Taylor and R. Soufti: “Specification, fabrication, testing, and mounting of EUVL optical substrates”, in EUV Lithography, SPIE Press Monograph, Vol. PM178, Ed.: Vivek Bakshi, 2008, p. 702). Further, for correcting aberration errors based on material removal well known methods are small tool polishing (J. S. Taylor, M. Piscotty, and A. Lindquist, Eds., “Fabrication and testing of aspheres, trends in optics and photonics (TOPS)”, Vol. XXIV, Optical Society of America, Washington D.C. (1999) and R. A. Jones, Ed., “Selected papers on computer controlled optical surfacing”, Vol. MS40, SPIE Press, Bellingham, Wash. (1991), ion beam figuring (F. Frost, R. Fechner, B. Ziberi, D. Flamm, and A. Schindler, “Large area smoothing of optical surfaces by low-energy ion beams”, Thin Solid Films 459, p. 100-105 (2004) and L. N. Allen and R. E. Keim, “An ion figuring system for large optics fabrication”, Proc. SPIE 1168, p. 33-50 (1989)) and plasma-assisted surface etching (S. J. Hoskins, “Aspheric surface figuring of fused silica plasma assisted chemical etching”, SPIE Vol. 2542, Optomechanical and Precision Instrument Design, p. 220-230 (1995) and the U.S. Pat. No. 6,858,537 B2).
The U.S. Pat. No. 6,844,272 B2 describes drawbacks of the methods mentioned above.
Several patents disclose various methods for controllable deformation of surface of solid material trying to overcome drawbacks mentioned above, which are described hereinafter.
The above mentioned U.S. Pat. No. 6,844,272 B2 discloses a method and apparatus for figure error correction on optical or other precision surfaces by changing the local density of material in a zone at or near the surface.
The U.S. Pat. No. 6,844,272 B2 gives the graph of the Mo/Si bilayer thickness dependence on the annealing temperature as an example for the implementation of the disclosed method. Excimer laser radiation may be applied for localized energy deposition into the predetermined region. Unfortunately, the invention does not disclose the way of laser radiation implementation, such as pulse width, pulse energy, focusing conditions, etc.
The very similar idea for repairing defects in a multilayer coating layered onto a reticle blank used in an EUVL system is described in United States patent with U.S. Pat. No. 6,821,682 B2.
The feasibility of the idea is illustrated by heating the Mo/Si multilayer with electron beam. However, electron beam implementation needs high vacuum conditions and has low throughput.
The idea of using laser radiation for aberration corrections of optical elements installed in the system is disclosed in U.S. Pat. No. 7,352,452 B2. This document is based on the known effect of compaction of some optical materials (for example—fused silica) under UV radiation (cf. e.g. U.S. Pat. No. 6,205,818 B1). The drawback of the U.S. Pat. No. 7,352,452 B2 is that some optical materials used in EUVL (for example, ZERODUR®) are not transparent for UV radiation, so the method can not be used for aberration corrections of reflective optical elements made of those materials. Another drawback of the invention is low throughput.
The US 2007/0224522 A1 describes a method for flattening a concave or convex substrate of an EUV substrate of a photolithographic mask by generating expanded portions at the corresponding positions of the substrate using a ultrashort pulse laser system. The US 2008/0032206 A1 discloses both the formation of a plurality of expansion stress generation portions and contraction stress generation portions again using an ultrashort pulse laser system. The expansion stress generation portions are generated using pulse durations in the range of 1 μs, whereas the contraction stress generation portions are formulated using laser pulses in the picosecond range. Both, expansion stress generation portions and contraction stress generation portions induce binding modification of the quartz lattice. However, this document does not disclose which processes occur in the stress generation portions. Therefore, it is not clear whether the expansion and contraction stress generation portions are temporally stable or whether the stress generation portions may damage the lattice.
The method of controllable bending of the surface of solid material based on the focusing of femtosecond laser pulses on the surface is described by P. Bechtold and M. Schmidt, “Non-thermal micro adjustment using ultrashort laser pulses”, JLMN-Journal of Laser Micro/Nanoengineering, Vol. 2, No. 3 p. 183-188 (2007). This paper considers two options. One option includes the ablation of pre-stressed coatings from a substrate, thus releasing the stress and producing the bending. Another option comprises micro-shockwaves inducing into the material with high-energy ultrashort laser pulses. After rapid transformation of the shock wave to the wave of compression they induce near-surface plastic deformation which results in bending of the surface.
Another approach for the modification of transparent dielectric materials includes focusing of ultrashort femtosecond laser pulses inside the material. Focusing the beam keeps its intensity below the damage threshold at the surface, but concentrates it enough at the focal point inside the material to cause multiphoton/avalanche ionization and structural changes of the material. In this case, near IR (infrared) (typically at about 800 nm or 1.06 μm) focused radiation is used to produce modified fields inside transparent material.
During last years this approach was used for three-dimensional data storage (E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Gallan, and E. Mazur, “Three-dimensional optical storage inside transparent materials”, Opt. Lett., Vol. 21, No. 24, p. 2023-2025 (1996), direct writing waveguides in transparent media (M. Ams, G. D. Marshall, P. Dekker, M. Dubov, V. M. Mezentsev, I. Bennion, and M. J. Withford, “Investigation of ultrashort laser-phonic material interaction: Challenges for directly written glass photonics”, IEEE J. of selected topics in quantum electronics, Vol. 14, No. 5, September/October 2008, p. 1370-1379), waveguide couplers writing (A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses”, Opt. Lett., Vol. 26, No. 1, p. 42-43 (2001)), and nanogratings producing (Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses”, Phys. Rev. Lett., Vol. 91, No. 24, p. 24705-1-247405-4, (2003), Y Shimotsuma et al., “Nano-modification inside transparent materials by femtosecond pulse laser”, Mod. Phys. Lett. B, Vol. 19, No. 5, p. 225-238, (2005), and R. S. Taylor, C. Hnatovsky, E. Simova, P. P. Rajeev, D. M. Rayner, and P. B. Corkum, “Femtosecond laser erasing and rewriting of self-organized planar nanocracks in fused silica glass”, Opt. Lett. B, Vol. 32, No. 19, p. 2888-2890, (2007)).
Non-linear photoionization of glasses leads also to the creation of laser-induced color centers. Color centers formation in fused silica and corresponding absorption spectra from DUV (deep ultraviolet) to near IR range are described by L. Skuja, H. Hosono, M. Hirano, “Laser-induced color centers in silica”, Proc. SPIE, Vol. 4347, p. 155-167 (2001). DUV attenuation was observed in the fields of modified fused silica, which were generated by ultrashort laser pulses at fluence below the threshold of laser-induced breakdown (S. Oshemkov, V. Dmitriev, E. Zait, and G. Ben-Zvi, “DUV attenuating structures in fused silica induced by ultrafast laser radiation”, Proc. CLEOE-IQEC, Munich 2007).
Summarizing the discussion of this section, there are some development efforts to understand the processes of non-linear photoionization in transparent dielectrics. However, presently there is no reliable and well understood method of using electromagnetic radiation for “polishing” optical surfaces in a controlled manner.
It is therefore one object of the present invention to provide a method and an apparatus for modifying a surface of a substrate of an EUV photolithographic mask that at least partially avoids the above-mentioned disadvantages.